Administration backs the server up, downloads, uploads and restores it

A Backups section (for administrators, PrivaPub's /clientapi/admin/backups): the backups kept with their kind, size,
media and the logins deleted since, what runs, the restore waiting and the last restore's report. Back up now; download
for the password through a link good for ten minutes, which the browser saves to disk; upload a downloaded backup in
pieces straight from the picked file (js/transfer.js PUTs file.slice at the offset PrivaPub has, never through .NET's
memory, and carries on after a refused or lost piece); delete after asking; restore for the password and the host typed
out, saying what it keeps and what it loses, then showing PrivaPub restarting until the ended session sends the page to
sign in.

e2e: a backup made, downloaded and uploaded again; and, only with --filter "Category=Restore", a real restore of the
pasture, which signs the town in again.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw

ClientJsonContext.cs

@@ -58,6 +58,12 @@ namespace decePubClient;
 [JsonSerializable(typeof(ViewStatisticsOverview))]
 [JsonSerializable(typeof(ViewHostsPage))]
 [JsonSerializable(typeof(ViewCrawler))]
+[JsonSerializable(typeof(ViewBackups))]
+[JsonSerializable(typeof(ViewBackup))]
+[JsonSerializable(typeof(BackupPasswordForm))]
+[JsonSerializable(typeof(RestoreBackupForm))]
+[JsonSerializable(typeof(ViewBackupTicket))]
+[JsonSerializable(typeof(ViewBackupUpload))]
 public partial class ClientJsonContext : JsonSerializerContext
 {
 }